The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in several IEEE standards documents, including for example, the IEEE Standard 802.11b (1999) and its updates and amendments, as well as the IEEE 802.15.3 Draft Standard (2003) and the IEEE 802.15.3c Draft D0.0 Standard, all of which are collectively incorporated herein fully by reference.
As one example, a type of a wireless network known as a wireless personal area network (WPAN) involves the interconnection of devices that are typically, but not necessarily, physically located closer together than wireless local area networks (WLANs) such as WLANs that conform to the IEEE Standard 802.11a. Recently, the interest and demand for particularly high data rates (e.g., in excess of 1 Gbps) in such networks has significantly increased. One approach to realizing high data rates in a WPAN is to use hundreds of MHz, or even several GHz, of bandwidth. For example, the unlicensed 60 GHz band provides one such possible range of operation.
In general, transmission systems compliant with the IEEE 802.15.3c or future IEEE 802.11ad standards support one or both of a Single Carrier (SC) mode of operation and an Orthogonal Frequency Division Multiplexing (OFDM) mode of operation to achieve higher data transmission rates. For example, a simple, low-power handheld device may operate only in the SC mode, a more complex device that supports a longer range of operation may operate only in the OFDM mode, and some dual-mode devices may switch between SC and OFDM modes. Additionally, devices operating in such systems may support a control mode of operation at the physical layer of the protocol stack, referred to herein as “control PHY.” Generally speaking, control PHY of a transmission system corresponds to the lowest data rate supported by each of the devices operating in the transmission system. Devices may transmit and receive control PHY frames to communicate basic control information such as beacon data or beamforming data, for example.
In wideband wireless communication systems that operate in the 60 GHz band, packets transmitted via a communication channel generally include a PHY preamble to provide synchronization and training information; a PHY header to provide the basic parameters of the physical layer such as length of the payload, modulation and coding method, etc.; and a PHY payload portion. A PHY preamble consistent with the IEEE 802.15.3c Draft D0.0 Standard, for example, includes a synchronization field (SYNC) that has several repetitions of a certain spreading sequence to indicate the beginning of a block of transmitted information, a start frame delimiter (SFD) field to signal the beginning of the actual frame, and a channel estimation sequence (CES) to carry information for receiver algorithms related to automatic gain control (AGC) setting, antenna diversity selection, timing acquisition, coarse frequency recovery, channel estimation, etc.
In general, antennas and, accordingly, associated effective wireless channels are highly directional at frequencies near or above 60 GHz. When multiple antennas are available at a transmitter, a receiver, or both, it may be important to apply efficient beam patterns to the antennas to better exploit spatial selectivity of the corresponding wireless channel. Generally speaking, beamforming or beamsteering creates a spatial gain pattern having one or more high gain lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, with reduced gain in other directions. If the gain pattern for multiple transmit antennas, for example, is configured to produce a high gain lobe in the direction of a receiver, better transmission reliability can be obtained over that obtained with an omni-directional transmission.
However, before a pair of devices complete a beamforming training session or estimate the communication channel between the devices so as to generate beamsteering vectors, typically one or both devices omni-directionally transmit control PHY data units (e.g., packets). Generally speaking, it is difficult for a device receiving control PHY data units in this manner to develop optimal or near-optimal antenna weights or obtain accurate frequency and/or timing synchronization.